The field of automatic backflushing fluid filters is replete with various designs that have been proposed over the years, both in the United States and abroad. Many of these designs employ a fluid filtering element which filters debris from a stream of fluid that is pumped through the element under pressure. Certain of these designs utilize a generally cylindrical filter element in which fluid flow passes from the interior of the filter element, through the element, to the exterior and out of the filter housing. As debris is collected upon the filtering or interior surface of the filter element, fluid flow through the filter is reduced and the pressure differential between the inlet and outlet of the filter rises, that is, the filter element becomes clogged with debris. Of those automatic backflushing filters having a generally cylindrical filter element, some are provided with a rotating backflush conduit located within the interior cylindrical surface of the filter element. The backflush conduit has an internal fluid passageway which serves as a receptacle for reversed fluid flow through the element during backflushing, such that debris collected on the interior peripheral surface is washed from the filter element through the backflush conduit and out to waste.
Typically, downstream pressure beyond the filter element is utilized to create a reverse flow through the element during back flushing. Namely, when a backflush conduit with an outlet which vents to atmospheric pressure is positioned in proximity to the interior peripheral surface of the filter element, the downstream head pressure exerted on the exterior of the screen element causes a reverse flow through the element into the low pressure backflush port. The reverse flow dislodges contaminants which are clogging the element. The foregoing mode of operation allows the automatic backflushing filter to be cleansed without disassembly and even without stopping the normal filtering operation of the filter, in that only the portion of the filter element proximate the inlet of the backflush conduit is reverse flushed to waste.
While the preceding description of known automatic backflushing apparatus has referred to the use of generally cylindrical filter elements, numerous other variations have been employed, such as truncated cones and rectangular filter elements over which reciprocates a backflush conduit. The teachings of the present invention are applicable to any of these variations as shall be apparent from the description of the invention which follows below. Similarly, automatic backflushing filters with cylindrical filter elements have been proposed wherein the fluid flow is from the exterior surface of the filter element to the interior. Designs of this type would be amenable to incorporating the teachings of the present invention as well.
Automatic backflushing filters have diverse uses in various environments, such as the filtering of waste water and working fluids in mills, factories, and sewage systems; filtering the particulates from crude oil; filtering cooling water for nuclear reactors; and in many other industrial, municipal and environmental applications, wherever fluid is processed by the removal of solids, particulates and debris. In those applications requiring the filtering of water taken from naturally occurring sources such as rivers, bays, lakes and streams, biological matter is frequently a prime contaminant. In many countries of the world, and more recently in the Americas, natural bodies of water have become the breeding ground for the zebra mussel, a small mollusk which has come to constitute a major concern with regard to its capacity to clog water supply lines and treatment facilities. The zebra mussel constitutes a particularly challenging problem for fluid systems engineers in that the mussel is very small, on the order of 40 Microns, but great in number. A contaminant of this size is especially hard to remove from automatic backflush filters, in that, to achieve a filter of pore size small enough to catch Zebra mussels, there is a decrease in open area ratio, i.e., the ratio of open area or pore area to closed area. A reduction in open area corresponds with an increased upstream/downstream pressure differential and a decrease in fluid flow, both in the forward and reverse directions. As a result, fluid processing rates are decreased and backflushing efficiency is deprecated. This necessitates frequent backflushing which translates to increased operational costs.
It is an objective in automatic backflush filter design to decrease the time that backflushing is carried out to prevent the loss of fluid to waste and to avoid unnecessary energy expenditure in moving the backflush conduit and diverting fluid pressure to backflushing rather than filtering purposes. It is also desirable for the backflushing operation to be effective in removing the maximum quantity of debris from the filter element. Ideally, all debris is cleaned from the filter element by backflushing. Known designs have employed various schemes for increasing the efficiency and effectiveness of the backflushing cycle in automatic backflushing filters. For example, scrapers, brushes, cutters and sprayers have been proposed as attachments to the backflush conduit for removing the debris cake from the filtering side of the element. Nevertheless, room for improvement exists.
Extensive effort has also been expended in producing filter elements or screens which facilitate filtering, have a long life, a reasonable cost, and promote effective backflushing. For example, convoluted perforated screens, elements having a plurality of stacked disc elements, elements made from wedge wire and perforated steel, have all been explored. Nevertheless, concerns relating to open area ratio and pore size limitations persist.
One limitation that is frequently present in automatic backflushing filters that arises from characteristics of the filter element, as well as the backflush conduit, is the failure of the backflushing apparatus to effectively and efficiently clean the entirety of the filter element. Typically, the flow pattern into the backflush conduit inlet port is not evenly distributed over the entirety of the port. In addition, flow through the port and conduit are constricted and turbulent, diminishing flow rate. As a result, the reverse flow through the filter element to the backflush outlet is not evenly distributed nor of maximum rate, leading to areas of the filter element that are not cleaned during backflushing. This condition is exacerbated by screens with poor backflush debris release characteristics and low open area ratio.
The present invention discloses a backflush conduit with an internal contour providing an enhanced flow therethrough leading to augmented reverse or backflush flow through the filter element during backflushing. This enhanced flow is more evenly distributed over the backflush conduit inlet port aperture such that the filter element is subjected to a more even cleaning during backflushing to avoid areas of ineffective or no cleaning of the filter element. Because there is an enhanced backflush flow though the backflush conduit, filter elements with smaller pore size and a lower open area ratio may be effectively used and cleaned with the backflush conduit of the present invention. This is particularly efficacious with respect to the removal of Zebra mussels. Backflushing is further facilitated by a filter element design with enhanced debris release characteristics.